The preferred method of removing mercury from coal-fired power plant flue gas streams is to inject a sorbent. The preferred sorbent is a porous carbonaceous char, typically an activated carbon. After the carbon is injected into the flue gas stream, it is captured by the particulate capture devices in the power plant and becomes part of the fly ash. Many utilities sell the ash to companies that service the cement and concrete industry which use the ash as a cement substitute. However, the presence of carbon in the fly ash can adversely affect the quality of cement and concrete that is made from fly ash since it tends to adsorb the foaming agent—commonly called an “AEA” or air-entrainment admixture—used in making the concrete. AEA's such as the vinsol resin MB-VR™, are used to create air voids in the concrete which make the concrete better able to contract and expand during freeze-thaw cycles. Without the air voids, the concrete is more likely to crack and spall. The problem, then, is that carbon, being a good sorbent, adsorbs and removes the foaming agent and thus tends to make the concrete more susceptible to cracking during freeze-thaw cycles. Sorbents other than carbon may be expected to produce similar outcomes. Excessive amounts of the foaming agent can be added to overcome this problem at the point of concrete manufacture, but the cost is often prohibitive and the testing the end-user must employ to determine the specific amount is time-consuming, ill-defined, and inconvenient. Where concrete-compatible carbons have been used in the prior art, they have either cost or supply limitations, such as Barnebey-Sutcliffe's type CA unimpregnated coconut or type CB (IAC) iodine-impregnated coconut, or they do not have sufficient mercury-removal capacity, such as RWE Rheinbraun's type HOK® lignite carbon.
To overcome this problem, a number of prior art solutions have been proposed, most of which depend upon post-treatment of the sorbent or the sorbent/fly ash by-product. Other art, such as that found in U.S. Pat. Nos. 4,453,978; 4,828,619; 5,110,362 and 5,654,352, has attempted unsuccessfully to resolve the problem by developing AEA's that are not adsorbed by the sorbent present in the fly ash. Still other art, particularly International Patent Application WO 2008/064360 A2 by Zhang et al., attempts to resolve the problem tautologically by identifying and selecting a concrete-compatible sorbent using a test, called the Acid Blue Index (ABI) (that serves merely as a surrogate for a test, called the Foam Index, long used for such purposes by the concrete industry), and calling the test a compositional parameter of the sorbent itself. Furthermore, Zhang et al. neither set nor claim boundaries for either the minimum level of surrogate test activity consistent with effective mercury removal or, indeed, the dimensional or compositional requirements of the sorbent for its primary function: the removal of mercury from the flue gas, a requirement that may conflict with the intended concrete-compatibility of the sorbent, producing sorbents with less than desirable commercial utility.
An example of sorbent post-treatment to resolve the problem may be found in U.S. Patent Application 20030206843/A1 by Nelson wherein a fly ash-friendly sorbent is made by oxidizing a carbonaceous sorbent with various chemicals, ozone in particular. A similar process has been proposed in U.S. Pat. No. 5,286,292 wherein the fly ash by-product itself is exposed to a halogen gas, preferably fluorine or chlorine, to neutralize the adsorption potential of the sorbent contained in the fly ash. However, such post-treatment processes are expensive, inconvenient, and often hazardous. Additionally, in the method of Nelson some surface oxygen groups may be stripped off at the high temperatures of the application, typically near or above 300F, rendering the treatment progressively ineffective during use. In U.S. Patent Application 20040069186/A1, P. S Zacarias and D. B. Oates propose oxidation of the fly ash by-product itself to gasify and remove any carbon that adsorbs or otherwise interferes with the AEA. Although directed primarily to sorptive carbon that entrains with the fly ash as part of the coal-burning process that produces the fly ash, the method could also be directed to fly ash by-products containing carbon sorbents added as mercury capture agents. In another variation of fly ash post-treatment, M. Tardiff, R. K. Majors, and R. L. Hill disclose in U.S. Patent Application 20040144287/A1 a process in which sacrificial agents, such as ethylene glycol phenyl ether and sodium di-isopropyl naphthalene sulfonate, are added to the fly ash to neutralize the adsorption of the AEA by sorptive carbons contained within the fly ash. A similar process is disclosed by R. D. Young in U.S. Patent Application 20040200389/A1. In U.S. Patent Applications 20070056479/A1 and 20070056481, L. J. Gray teaches the addition of a fluorochemical surfactant to the concrete during make-up that can preferentially stabilize the foam created by the AEA, even for fly ash containing up to 6% sorptive carbon as measured by loss on ignition (LOI), a measure of fly ash carbon content used customarily in the cement and concrete industries (ASTM C618-01). Once again, however, these post-treatment methods represent added cost and inconvenience. Other mercury sorbents, particularly those made from mineral materials, may be more fly ash-friendly, but lack the capability to efficiently remove mercury and other contaminants from the flue gas, making carbon-based sorbents by far the preferred sorbents for mercury capture and removal.
Therefore, it is an object of the present invention to provide a sorbent that efficiently and effectively removes mercury and/or other contaminants from flue gas streams while retaining the value of the fly ash for commercial cement and concrete applications. It is further the object of the present invention that both the contaminant adsorptive properties and concrete-compatible properties be provided as an integral and inherent feature of the sorbent's structure as manufactured, independent of any additional post-treatments or processes. Unlike the prior art, the present invention affords measurable upper and lower dimensions of pore surface areas, volumes, and diameters to provide optimal contaminant removal and concrete compatibility.